1. Field of the Invention
This invention relates to a process for effecting mass transfer between a liquid phase and a gaseous phase in a filled-type column wherethrough said phases are caused to flow in a countercurrent relationship.
In the description given below and in the following claims, the term: xe2x80x9cfilled-type columnxe2x80x9d, is understood to meanxe2x80x94in generalxe2x80x94an apparatus having, disposed on its interior, a plurality of variously shaped and sized elements (filler) at whose surfaces a liquid phase and a gaseous phase are caused to contact each other in order to effect mass transfer; apparatus of this type are widely employed in chemical plants, e.g. for decomposing, absorbing, distilling, and scrubbing chemicals.
The invention also relates to a filled-type column for implementing the above process, and to a method of retrofitting a pre-existing column for conversion into a filled-type column according to the invention.
In the description given below and in the following claims, the term: xe2x80x9cretrofittingxe2x80x9d, is understood to mean the in-situ modification of a pre-existing column of the filled or plates type to improve its performance and, for example, to increase its capacity and/or its efficiency in mass transfer between the liquid phase and the gas phase, as well as to reduce its energy consumption.
As is known, in the field there is increasingly felt the need of providing processes easy to implement, which can effect mass transfer between a liquid phase and a gaseous phase in a simple and efficient way, at low investment and operating costs and with a low energy consumption.
2. Prior Art
In order to meet the above requirement, processes for effecting mass transfer have been proposed in the field wherein a liquid phase and a gaseous phase are caused to flow through a filled-type column in a downward and an upward (substantially axial) direction, respectively.
While being simple to implement, such prior processes are beset with problems arising mainly from the large pressure drop experienced by the gas phase in going through the filler.
The filler is, in fact, disposed inside substantially cylindrical columns having a large height-to-diameter ratio intended to ensure an interphase contact of sufficient duration for the phases to enhance the mass transfer.
Consequently, in flowing through the filler, the gas phase undergoes a significant reduction in pressure (pressure drop), which places limitations on the amount of gas that can be fed into the column, thus curtailing the column capacity.
By reason of this pressure drop, the flow rate of the gas phase fed into the column must be held below predetermined values, the overtaking of which would result in the appearance of an undesirable xe2x80x9cfloodingxe2x80x9d phenomenon, whereby the column becomes flooded with liquid phase which is held back in its downward movement by frictional drag from the gas phase. It will be appreciated that in this condition the column would no longer be operable, due to the mass transfer dropping to virtually nil.
In other words, the large pressure drop of the gaseous phase in going through the filler is a limiting factor of the gas phase flow rate through the column designed to implement the above prior art processes, and hinders an effective mass transfer between the phases.
To counteract the pressure drop in the gaseous phase, and thus to increase the capacity of filled-type columns according to the prior art, some special fillers having a high void ratio have been proposed, wherethrough the gaseous phase stream undergoes no large pressure drops.
While such fillers allow the flow rate of the gas phase being fed into the column to be increased somewhat, they have been unsuccessful in providing a fully satisfactory mass transfer between the phases, because the pressure drop of the gas phase through the column is still quite significant, as are the operational constraints due to the aforementioned flooding phenomenon.
Columns of the so-called plates type, i.e. provided with a plurality of perforated plates fitted horizontally inside the column, have been proposed alternatively to the filled-type columns.
In this case, the process for effecting mass transfer between the liquid and gaseous phases comprises mixing the phases together in the plates, the plates being typically designed to enhance the mutual contact of the phases. Plates-type columns have been found suitable for low liquid flow-rate applications, but are liable to become flooded, disallowing any further effective mass transfer between the liquid and gas phases.
It is on account of the above disadvantages that prior art processes for effecting mass transfer between a liquid phase and a gaseous phase have provided unsatisfactory performance heretofore, both in terms of overall efficiency of the transfer between the phases, energy consumption, and operating and investment costs of the columns used to implement such processes (which are, as said before, beset with several limitations).
All this, despite this technology having been utilized in a variety of chemical applications for years and the aforementioned need being increasingly felt in the field.
The problem underlying the present invention is that of providing a process for effecting mass transfer between a liquid phase and a gaseous phase, which process affords a highly efficient transfer between the phases in a simple and effective way, at low investment and operating costs and with low energy consumption.
This problem is solved, according to the invention, by a process as indicated above for effecting mass transfer between a liquid phase and a gaseous phase within a filled-type column which comprises an external shell accommodating at least one filler-containing basket wherethrough said phases are caused to flow in countercurrent, which process is characterized in that it comprises the step of feeding said gaseous phase to said at least one basket through a gas-permeable basket surface which is larger than the basket cross-section.
In this way, by causing the gaseous phase to flow through a permeation surface of said at least one basket which is advantageously made larger than the basket cross-section, a corresponding reduction is obtained in the pressure drop of said phase flowing through the filler, which allows the gas flow rate to be increased andxe2x80x94at the same timexe2x80x94to operate at slower velocities than prior art processes, thereby significantly enhancing the mass transfer between the phases.
The benefits in terms of improved efficiency of mass transfer are the more substantial, the larger the permeation surface presented to the gaseous phase.
In this respect, the above problem is solved, in a particular effective way, by a process for effecting mass transfer between a liquid phase and a gaseous phase within a filled-type column which comprises an external shell accommodating at least one filler-containing basket whose cross-section is smaller than the cross-section of said shell, said process comprising the steps of:
feeding said liquid phase and gaseous phase into said filled-type column;
causing the liquid phase to flow through said at least one basket in a substantially axial direction;
causing the gaseous phase to flow through said at least one basket in a prevailing radial direction;
extracting said liquid phase and gaseous phase from said filled-type column.
With the process of this invention, and particularly by causing the gaseous phase to flow through the filler in a prevailing radial directionxe2x80x94rather than in an axial direction as taught by prior art processesxe2x80x94the permeation or passage surface can be drastically expanded to positively enhance in a simple and effective way the mass transfer between the phases and, hence, to significantly increase the capacity of the column designed to implement such a process over a column of comparable size operating according the aforementioned prior art processes.
Stated otherwise, for a given capacity, the column implementing the process of this invention can by made substantially smaller than a prior art column.
This arises mainly from that the pressure drop undergone by the gas phase in flowing radially through the filler can be considered trivial compared to the pressure drop involved in going through the filler in the axial direction, so that the process of this invention can advantageously be carried out at higher gas flow rates than in the prior art, before such undesirable phenomena as flooding occur, thereby enhancing the mass transfer between the liquid and the gaseous phase.
In particular, the process of this invention can virtually eliminatexe2x80x94in an extremely simple and effective wayxe2x80x94the constraints of pressure drop and limited flow-rate of the gas phase through the column, allowing to operate at desired volumes and velocities of the gas and liquid flows for optimum utilization of the exchange surface area of the filler for effecting mass transfer.
Advantageously, thanks to this process, a highly efficient mass transfer between the phases can now be achieved using a filled-type column which performs most efficiently, and involves low investment and operating costs and at low energy consumption.
The fact that the mass transfer between the phases can be significantly improved by flowing the gas phase in a prevailing radial direction through the filler, is in sharp contradiction with the constant prior art teaching that a crossflow of the gaseous phase relative to the liquid phase is less advantageous, from the standpoint of the mass transfer, than an axial flow of the phases in true countercurrent relationship.
In addition, since the conditions and the chemical/physical processes which are responsible for the mass transfer between the phases can vary substantially with the flow type, the benefits of the process according to the present invention were not at all predictable a priori.
It was only the research work carried out by the Applicant that unexpectedly brought out how, by having the gaseous phase flow in a prevailing radial direction, in countercurrent to the substantially axial flow of the liquid phase, the driving force responsible for the mass transfer between the phases could act much more efficiently than in a true countercurrent flow, and thus the effectiveness and intensity of the mass transfer could be advantageously augmented.
Particularly satisfactory results have been obtained by causing the gas phase to flow through said at least one basket in substantially radial, substantially axial-radial, or substantially crosswise directions.
Preferably, the prevailing radial flow of the gaseous phase through said at least one basket is of an either centrifugal, or centripetal, or alternating centrifugal and centripetal type.
In a particularly advantageous embodiment of the invention, the liquid phase and the gas phase are caused to flow in said at least one basket through a plurality of contiguously superimposed zones, with the liquid phase and the gas phase being fed to an upper zone and a lower zone, respectively, of said at least one basket, and extracted from a lower zone and an upper zone thereof, respectively.
By dividing the filler into a plurality of zones to be traversed by the gas phase in a prevailing radial direction, it has been obtained an increase in the radial component of the gas flow, with the consequence that it is possible to further augment the flow rate of the gas phase and thus improve the mass transfer between the phases and with them, the capacity of the column used to implement this process.
Alternatively, the same result can be obtained by causing the liquid phase and gas phase to flow in a substantially axial, respectively prevailing radial direction through a plurality of superimposed filler-containing baskets.
Advantageously, according to the last-mentioned embodiment, the process according to the invention further comprises the step of:
collecting and re-distributing said liquid phase between successive baskets.
In this way, it is possible to maintain the whole filler constantly swept by the liquid phase, thus avoiding the latter from flowing down along preferential paths which swept only local zones of the filler, resulting in a much reduced effective surface area being available for the mass transfer.
Preferably, the gas phase is caused to flow radially through contiguous zones of said at least one basket or through successive baskets, in opposite directions, thereby to provide a zigzag flowpath for the gas phase within the column, which proves advantageous from the constructional standpoint.
For implementing the above process, the invention advantageously provides a filled-type column for effecting mass transfer between a liquid phase and a gaseous phase, which comprises:
an external shell;
at least one basket for containing the filler extending inside said shell, said at least one basket being through-penetrated by said phases in countercurrent relationship;
respective means for feeding said liquid phase and said gaseous phase into said column;
respective means for extracting said liquid phase and said gaseous phase from said column; the column being characterized in that said at least one basket is provided with a gas-permeable surface, for passing the gaseous phase, which is larger than its cross-section.
Preferably, the column according to the invention is characterized in that said at least one basket has a smaller cross-section than the shell cross-section and has opposite sidewalls which are gas-permeable, and in that it further comprises means for causing said gaseous phase to flow through said at least one basket in a prevailing radial flow.
According to a preferred embodiment, the column advantageously comprises:
an external shell, substantially cylindrical in shape;
at least one annular basket for containing the filler which is disposed coaxially within said shell and includes opposite respectively inner and outer gas-permeable cylindrical walls, said at least one basket being through-penetrated by said liquid phase in a substantially axial flow direction;
a first free space defined between an inner wall of the shell and said outer wall of the basket;
a second free space defined inwardly of said inner wall of the basket;
respective means for feeding said liquid phase and said gaseous phase into said column;
means for causing at least a major portion of said gaseous phase to flow through said at least one basket, from said first free space to said second free space, or the other way round;
respective means for extracting said liquid phase and said gaseous phase from said column.
According to a further preferred embodiment, the column advantageously comprises:
an external shell;
at least one basket for containing the filler, disposed inside said shell coaxially therewith and provided with opposite sidewalls, preferably plane, which are gas-permeable, said at least one basket being through-penetrated by said liquid phase in a substantially axial flow direction;
first and second free spaces, located opposite to each other and defined between an inner wall of said shell and said sidewalls of the basket;
respective means for feeding said liquid phase and said gaseous phase into said column;
means for causing at least a major portion of said gaseous phase to flow through said at least one basket, from said first free space to said second free space or the other way round;
respective means for extracting said liquid phase and said gaseous phase from said column.
According to a further aspect, the invention provides a method of retrofitting a column for effecting mass transfer between a liquid phase and a gaseous phase, being of either the filled or the plates type, which method is characterized in that it comprises the step of:
providing, inside said column, at least one basket for containing a filler and having a gas-permeable surface, for passing the gaseous phase, which is larger than its cross-section.
Thanks to the above method of retrofitting an existing column, a process for effecting mass transfer between a liquid phase and a gaseous phase can be obtained which allows to provide a high rate of transfer between the S phases in a simple and effective manner, at low investment and operating costs, and with low energy consumption.
Further features and advantages of this invention will become apparent from the following description of an embodiment of the process according to the invention, given by way of non-limitative example with reference to the accompanying drawings.